1. Field of the Invention
The present invention generally relates to the art of processing hydrocarbon compositions, and more particularly to a method for cracking hydrocarbon compositions.
2. Description of Related Art
The processing of hydrocarbon compositions to manufacture low molecular weight/lower boiling point organic products is commonly known as "cracking". Hydrocarbon cracking processes are widely used in many different technical fields, such as in the production of speciality organic chemicals and, with particular importance, in the petroleum processing industry.
An important product obtained from petroleum is gasoline, which is mainly used as motor fuel. Gasoline is a complex mixture including hundreds of different hydrocarbons containing 4 to 12 carbon atoms per molecule (the range may slightly vary depending on the source of definition). The different hydrocarbons have very different structures effecting the quality of fuel. It is known that the higher the degree of branching of the hydrocarbon chains, the higher the quality of the fuel and the less frequently the so-called "engine-knock phenomenon" occurs.
Since the amount of gasoline directly obtained by fractional distillation from refinery does not satisfy the need for its primary use as liquid fuel, thermal cracking, and later, catalytic cracking of crude oil, in particular heavy oil have been applied to increase the production of gasoline. Various methods of catalytic cracking are known of which FCC (Fluidized Catalytic Cracking) has become a very important operation for cracking of hydrocarbon compositions. Typical processes are using a fluidized bed of a particulate carrier/catalyst composition generally in the presence of hydrogen gas under pressure. Acid silicate catalysts including but not limited to silica-alumina-nickel as well as other comparable catalytic agents such as zeolites are commonly used as catalysts. The zeolite ZSM-5 has been recently found to be the best catalyst for FCC since this zeolite leads to most selective gasoline production thanks to its shape selectively. Light alkenes, in particular C.sub.3 and C.sub.4, and gas oil are the major secondary products with this catalyst.
A major problem with respect to the FCC process, however, is the coke information and carbon deposit on catalyst leading to a deactivation of the latter. Therefore, the catalyst has to be put into a regenerator to remove the carbon deposit and coke immediately after the FCC reaction causing a decrease of profitableness of the FCC processes.
More recently, plasmas have been found to be versatile tool for the development of new industrial processes and products. The properties of plasmas can be modified and a distinction is made between thermal and nonthermal plasmas differing markedly in both discharge characteristics and applications.
The energy distribution of the gas molecules, ions and electrons in thermal plasma indicates that the system is in thermal equilibrium and thus close to thermodynamic equilibrium. The temperature in the discharge region is uniformly very high for all particles. Moreover, there is a high energy flux in the plasma volume as well as at the electrodes if present. Thermal plasmas are therefore often called "hot plasmas". Hot plasmas include, in particular, arc discharges.
An essential condition for the formation of a thermal plasma is a sufficiently high working pressure usually being over 10 kPa. The resulting large number of collisions between particles, in particular between electrons and heavy positive ions or neutral particles, leads to rapid redistribution of energy so that equilibrium is reached.
Nonthermal plasmas, in contrast, are far from thermodynamic equilibrium. Nonthermal plasmas have comparatively low gas temperature and energy-conversion rates. Thus, the electrons in these plasmas have typically a very much higher temperature than the heavy ions and neutral particles. Nonthermal plasmas are therefore also named "cold plasmas". This group typically includes glow and silent discharges as well as radio-frequency and microwave discharges at pressures below 10 kPa. The feasibility of cold plasma has been confirmed by the industrial production of ozone. For brevity, reference is made to a report of Eliasson et al. in IEEE Transactions on Plasma Science, Vol. 19, page 1063-1077, the disclosure of which is incorporated herein for all purposes by way of reference.
The use of thermal plasma discharges for heavy hydrocarbon cracking, aromatics conversion and fuel upgrading pyrolysis has been reported. Thus, J. L. Leuenberger et al. has developed a thermal plasma hydrocracking process using an argon hydrogen plasma torch (report of J. L. Leuenberger, M. Mohammedi, E. Fraricke and J. Amouroux in Proc. of 12.sup.th Int. Symp. on Plasma Chemistry, Minneapolis, USA, V.11, pp. 595-600, Aug. 21-25, 1995; this report being incorporated herein for all purposes by way of reference).
Moreover, U.S. Pat. No. 5,626,726 discloses a method using a thermal plasma for cracking a liquid hydrocarbon composition, such as crude oil, to produce a cracked hydrocarbon product. An electrical arc is generated directly within the liquid hydrocarbon composition so that the arc is entirely submerged in the composition. Arc generation is preferably accomplished using a primary and secondary electrode each having a first end submerged in the composition. The first ends of the electrodes are separated from each other to form a gap therebetween. An electrical potential is then applied to the electrodes to generate the arc within the gap. A reactive gas is thereafter delivered to the arc which forms a bubble around the arc. The arc and gas cooperate to produce a plasma which cracks the hydrocarbon composition.
Cracking of hydrocarbons via thermal plasma, however, is typically an intensive high temperature process and often requires, as in the abovementioned cases, an extra immediate quenching step to avoid production of carbon deposit and to get a sufficiently high quality of products. This induces a complex system. A lot of energy is thereby consumed that reduces the energy-efficiency. Moreover, the selectivity of products is not easy to control with thermal plasma processes so that further refining steps are often necessary for obtaining high quality products. Such quenching and/or refining steps lead to a significant increase in cost of manufacture.
In addition, safety regulations are often decisive whether a new developed process will be industrially applied. Hydrocarbon cracking processes operating via thermal plasmas generated by arc discharges cause the danger of spark flash-overs. Therefore, petroleum industry generally tend to avoid the incorporation of such processes within its pool of manufacturing methods.
Moreover, processing as well as apparatus requirements are not always easily met and there exists a continuous need for improved methods and apparatuses for cracking hydrocarbon compositions into lower molecular weight/lower boiling point products suitable for use as liquid fuels.